There is increasing demand for clean and reliable sources of energy. It is widely believed that at least part of the solution lies in improved technologies for storing energy, such as batteries and electrochemical capacitors, in addition to technologies for generating energy, such as photovoltaic cells and photocatalytic devices. With respect to all of these technologies, there is a desire that any such devices be both compact and efficient.
Research into these fields has been intensive and is ongoing. For example, the photocatalytic activity of nanostructured semiconductor films has been widely explored in designing solar cells, solar hydrogen production, and environmental remediation (see, for example, P. V. Kamat, Chem. Rev. 93, 267 (1993); P. V. Kamat, J. Phys. Chem. C 111, 2834 (2007); and A. J. Bard et al., Acc. Chem. Res. 28, 141 (1995)). An ongoing challenge in attaining higher photoconversion efficiency is how to promote the transport of electrons to the collecting electrode surface without recombination events that would otherwise reduce device efficiency. To this end, various composite materials have been investigated (see, for example, A. Kongkanand, et al. Nano Lett. 7, 676 (2007)). Nevertheless, significant challenges remain, including the need to construct nanostructures having acceptable dimensions over a large area.